The present invention relates to a nuclear reactor power control method and a nuclear reactor power control device, in particular, for applying to a boiling water type nuclear reactor.
The boiling water type nuclear reactor comprises a feed water system including a turbine driven reactor feed water pump (TDRFP) and a motor driven reactor feed water pump (MD-RFP). In normal operation, with two turbine driven reactor feed water pumps, the feed water is supplied to the reactor. The motor driven reactor feed water pump is also used as a back-up for the turbine driven reactor feed water pump. When one turbine driven reactor feed water pump is abnormally tripped and the motor driven feed water pump is not started, the flow of feed water becomes short and a nuclear reactor scram is caused due to the decrease of water level of the nuclear reactor. Scram can be considered as a general plant trip.
It is known to control the power of a nuclear reactor by changing the flow of the recirculated water. This controls the amount of voids in the water between fuel elements and a void will absorb neutrons to a greater extent than water. Therefore a reduction in the recirculated water by reducing the speed of the core cooling fluid pump will increase the amount of voids and thus decrease the power.
A method for avoiding such nuclear reactor scram is explained in Japanese Patent Application Laid-Open No. 55114998(1980). When both the water level of the nuclear reactor decreases below a fixed level and a mismatching between the flow of the feed water supplied to the nuclear reactor and the flow of the steam discharged from the nuclear reactor appears, the method directly controls a recirculation flow control device with the mismatched amount and thereby decreases the nuclear reactor power. Thereafter, the flow of the steam decreases and the water level of the nuclear reactor rises. Thereby the nuclear reactor scram is avoided. This method detects the shortage of feed water flow due to non-start of the motor driven reactor feed water pump by measuring the water level drop of the nuclear reactor.
Further, although different from the abnormality example of the above feed water pump trip, Japanese Patent Application Laid-Open No. 49-37094 (1974) discloses a method for avoiding the nuclear reactor scram caused by the water level decrease of the nuclear reactor due to the decrease of feed water flow. The objective deals with the problem when there is a decrease of the feed water flow due to breakage of the feed water piping. The method is to lower the nuclear reactor power when both the feed water flow decreases below the steam flow by more than a fixed value and the water level of the nuclear reactor decreases from a normal water level by more than a fixed value. As the operations for lowering the nuclear reactor power, there are decreasing the speed of the recirculation pump and the insertion of selected control rods.